DESCRIPTION (adapted from the applicant's description): The phosphorylation state of the largest subunit of RNA polymerase II determines its ability to be recruited into a preinitiation complex or carry out efficient RNA chain elongation. Further, the C-terminal heptapeptide repeat domain (CTD) that is the target for phosphorylation is essential for several additional steps in mRNA biogenesis, including mRNA capping, splicing, and 3' end processing. Thus it is likely that CTD phosphorylation regulates all of these processes in the cell. Understanding the mechanisms that govern CTD phosphorylation and dephosphorylation is critical to learning how transcription is coordinated with other steps in nuclear RNA metabolism. Numerous protein kinases capable of phosphorylating the CTD on different sites are known, but CTD-specific phosphatases have not been described until recently, with the purification and cloning of a yeast serine/threonine phosphatase subunit capable of this reaction, and the discovery of a mammalian homologue. The phosphatase subunit encoded by the essential FCP1 gene is not sufficient for CTD dephosphorylation activity in vitro, and an unknown positive cofactor (FcpX) has been implicated. Further, the phosphatase activity is inhibited by the HIV Tat protein, which is known to stimulate elongation by the hyperphosphorylated form of RNA polymerase II. The aims of this research are to: (1) Isolate and characterize the cofactor FcpX; (2) Carry out a structure-function analysis of FCP1 using yeast genetics and biochemistry; and (3) determine the role(s) of Fcp1 in the cell using genetic suppressor analysis combined with biochemistry.